Carbon Dioxide-Based Neutralization of High-Density Sludge Effluents as a Sustainable Climate and Water Quality Alternative to the Use of Strong Mineral Acids

The neutralization of high-density sludge (HDS) effluent is a required process involved in the treatment of acid mine drainage (AMD). In their last treatment stage, effluents with high pH values are acidified to reach legal standards before being released to the environment using hydrochloric or sulfuric acid. In this investigation, CO2 was tested as an alternative way to decrease the pH of the HDS effluent, together with an economic analysis comparing the results with the use of strong mineral acids, considering a full-scale 300 m3/h plant. HDS samples were collected from a PAN American Silver operation in Cajamarca, northern Peru. Four acidification tests were carried out on 20 L containers, with a subsequent evaluation of reaction time and CO2 consumption to regulate the final pH of the treated solution. The results suggest that by adding CO2 (0.5 L/min) to the solution, the pH was successfully decreased from 10–10.5 to 6.5–7.5 (which falls within the legal limits) in a matter of minutes. An average of 130 g of CO2 was sequestrated per m3 of solution to decrease the pH within legal limits, representing around USD 0.031/m3 in terms of treatment cost for a full-scale plant. While this is more expensive than using other acids, with a CO2 credit of USD 100/ton, sequestrated CO2 neutralization is 12% cheaper and only 6% more expensive than using H2SO4 and HCl, respectively. Moreover, in terms of the costs per ton of avoided CO2 of USD 133 and USD 262 for replacing hydrochloric and sulfuric acid, respectively, it is markedly lower than the cost of other CO2 abatement technologies, like, for instance, solar photovoltaic panels (PV) that can cost between USD 368 and USD 684/ton of avoided CO2 in Peru and require substantial capital investments. Moreover, the use of CO2 implicates a series of additional safety, operational, and environmental advantages that should be considered. Therefore, the use of CO2 to decrease HDS effluent’s pH should be further explored in Peru and elsewhere as a sustainable alternative.